There are many examples of devices for scanning an original to produce video signals. Many of these devices produce line graphics or halftones by imaging separate patterns. Such an example is shown in U.S. Pat. No. 3,806,641 of Crooks where character words are stored for reproduction of image areas within the original.
In the prior art devices, a series of line graphic or halftone dot patterns may be reproduced as separate image areas. A succession of separate image areas may form one or more lines of images. In most cases, successive lines are formed in a raster by reproducing a succession of image areas in a line and deflecting or leading the imaging means relative to the imaging surface in the direction of the succession of lines. Where the beam is deflected a maximum in the direction of successive raster lines and the maximum beam deflection is less than the length or width of the graphic image, then it is necessary to increment the imaging surface, to bring a fresh portion of the imaging surface under the beam. When this is done, the beam is deflected in the opposite direction to its starting position and swept across the imaging surface in a raster to form a new succession of lines.
A beam can be leaded with a much greater precision than the imaging surface can be leaded by a mechanical movement. A leading space between image lines, formed by movement of the beam will usually be consistent with all other leading spaces produced by movement of the beam. However the leading space resultant from mechanically moving the imaging surface cannot be reproduced with the same precision as the leading space resultant from deflection of the beam. The mechanical movement for moving an imaging surface cannot be started and engaged, and then stopped with the precision of the beam deflection means nor can leading space resultant from the mechanical movement be reproduced with sufficient precision when leading the imaging surface a second time. As a consequence, the leading spaces between lines formed by movement of the imaging surface can be of varying widths compared to each other and of a different width compared to the leading space produced by movement of the imaging beam.
The difference in width is known as leading space error. It is possible for the human eye to detect very small differences in leading space error resultant from leading of the imaging beam from line to line and leading of the imaging surface. It is very difficult and expensive to reduce this error below the very small differences that can be detected by the human eye. Additionally, a highly precision movement for the imaging surface is normally not found in typesetters designed only for text output where interline spacing does not have such a detrimental effect upon the appearance of the text as it does in the reproduction of halftone dots.